This invention relates to a novel process for the recovery of acetic acid from dilute aqueous streams. In particular, this invention relates to such recovery from dilute aqueous streams which have been formed during the carbonylation of methanol or methyl acetate in the presence of a Group VIII metal carbonylation catalyst to acetic acid and to the purification thereof. More specifically, this invention pertains to a novel process for separating acetic acid from dilute aqueous streams and using the purified aqueous stream in acetic acid purification such as for removing alkane impurities from an acetic acid product formed by Group VIII metal catalyzed carbonylation processes.
Among currently-employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium, or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior-art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. No. 5,001,259, issued Jun. 25, 1991, and European patent application 161,874; published Nov. 21, 1985. As disclosed therein acetic acid is produced from methanol in a reaction medium comprising methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically-effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 12 wt. % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically-effective amount of rhodium, at least a finite concentration of water, methyl acetate, and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. These patent publications teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product-recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. No. 5,001,259 is herein incorporated by reference.
The acetic acid which is formed by the carbonylation of methanol is converted to a high purity product by conventional means such as by a series of distillations. During the distillations to purify the acetic acid product, there are obtained aqueous streams which contain minor amounts of acetic acid, organic iodide co-catalyst and methyl acetate which are recycled to the carbonylation reactor. Since it is advantageous at least as far as increasing production of acetic acid to reduce the water content in the carbonylation reactor to well below 12% and, preferably, approach 3-4 wt. % water in the carbonylation reactor, the recycle of these aqueous streams from the purification process renders the management of the water level in the reactor problematic. It is not efficient to simply discard these aqueous streams since valuable acetic acid product would be discarded as well as necessary reactants which would have to be made up by new and more expensive supplies.
During the purification of the acetic acid formed by the above-mentioned carbonylation, it is known to utilize aqueous streams to aid in the removal of impurities from the acetic acid product. Thus, it has been found that under the low water carbonylation conditions the acetic acid which is produced under such conditions is deficient in permanganate time. Among the impurities which degrade permanganate time of the acetic acid are carbonyls and organic iodides. In EPO Published Application 487,284, published May 5, 1992, is disclosed a process to minimize the amount of circulating carbonyl-containing and unsaturated organic materials in the carbonylation reaction mixture resulting in a more facile purification of acetic acid. By such process, the carbonyl impurities are reacted with an amine compound to form oximes wherein the oximes are removed by distillation including the addition of water to extract or otherwise solvate the oximes in an aqueous phase.
Another purification process which involves the removal of impurities from the acetic acid product involves the removal of alkanes from the crude acetic acid product. The formation of alkanes was recognized by Price. His invention, described and claimed in U.S. Pat. No. 4,102,922, involved removing the alkanes by stripping the volitile components of the reaction away from the alkanes after removal of the catalyst. The reaction mixture is carried to a pressure let-down vessel denoted as a flasher where the products are vaporized and removed from a residue of catalyst. The catalyst is recycled to the reactor. The flashed product containing methyl iodide, water, acetic acid, and the alkanes is fed to a splitter and allowed to separate into at least two liquid phases, one phase containing acetic acid and water which is returned to the reactor and a second phase denoted herewithin as a heavy phase. To effect removal of the alkanes, a slipstream of the heavy phase from the splitter column is stripped using carbon monoxide as a stripping gas, removing the alkanes as the bottoms stream from the latter distillation.
Disclosed in commonly assigned U.S. Pat. No. 5,371,286, is a method by which the alkanes are removed according to the method of Price but with the unexpected benefit of realizing the recovery of acetic acid from the residue that would normally be discarded by the practice of the prior art. In a preferred embodiment of the invention, a method is provided for effecting the alkanes removal at low-water conditions wherein the water balance in the reaction system is maintained.
As disclosed therein, the carbonylation of methanol, dimethyl ether, methyl acetate, or a mixture thereof takes place in the presence of a Group VIII metal carbonylation catalyst. Such carbonylation reactions comprise catalytic reaction with carbon monoxide in the presence of a halide promoter such as an organic halide as disclosed in U.S. Pat. No. 3,769,329 or under low water conditions such as disclosed in aforementioned U.S. Pat. No. 5,001,259 wherein the catalyst solution contains not only the Group VIII metal catalyst and organic halide promoter, but also contains an additional iodide salt. In such processes, a feed of methanol, dimethyl ether, methyl acetate, or mixture thereof is carbonylated in a liquid phase carbonylation reactor. Separation of products is achieved by directing the contents of the reactor to a flasher wherein the catalyst solution is withdrawn as a base stream and recycled to the reactor while the overhead which comprises largely the product acetic acid along with methyl iodide, methyl acetate, and water is directed to a methyl iodide-acetic acid splitter column. The overhead from the splitter column comprises mainly organic iodides and methyl acetate whereas from the base or side stream of the splitter column is drawn the acetic acid product which is usually directed to further purification by finishing distillation. It is a portion of the overhead, heavy phase, that contains a majority of the alkanes which are removed by the process of the invention disclosed therein. Thus, the heavy phase from the splitter column overhead is distilled in a refluxing column at a reflux ratio of from about 0.5 to about 5. The overhead from this refluxing column, containing methyl iodide, methyl acetate, and carbonyl impurities, is removed and returned to the reactor or treated further to remove the carbonyl impurities. The residue from this column is passed to a decanter where the phases are caused to separate by the addition of water. The bottom phase being substantially water and acetic acid is returned to the reactor. The upper phase being substantially all of the alkanes is waste and can be used as feed to an incinerator or the like for disposal. In a preferred embodiment of the invention disclosed in U.S. Ser. No. 052,429, the phases are caused to separate by the addition of a slipstream of the overhead from the acetic acid drying column further down in the purification train. The drying column overhead is first distilled to separate a methyl acetate and methyl iodide distillate from the aqueous bottoms. The aqueous bottoms derived from distilling the drying column overhead is used to provide water for the phase separation and some water balance in the carbonylation reaction system.
Unfortunately, the slipstream from the drying column overhead which is utilized can contain up to 20 wt. % acetic acid which remains in the aqueous bottom phase during distillation to remove the methyl acetate and methyl iodide. Removal of the acetic acid from this stream will make it much more effective in the phase separation of the alkanes from the aqueous phase. Moreover, if too much water is present in the system, purging the aqueous stream without effectively removing acetic acid product therefrom is not efficient as valuable acetic acid product is discarded. Methods of separating the acetic acid from the aqueous phase such as by distillation or solvent extraction are not cost effective in view of the energy costs involved in the distillation process and the need for high capital investment if solvent extraction is utilized.
Accordingly, there is a need to control the water levels in the carbonylation system, in particular, when the water concentration in the carbonylation reactor is to be maintained below about 12 wt. %.
Further, there is a need to treat other dilute acid aqueous streams in the carbonylation separation and purification process to separate the acetic acid from the aqueous phase in a cost efficient manner. As for example, the light phase from the stripper column contains acetic acid and water which can be used to cause the phase separation described above were the acetic acid and water separated. The water recycle could then be more easily controlled without the concommitant loss of acetic acid.
As the concentration of water in the reactor is lowered to less than 12 weight percent range the inventors have found that there are additional needs to treat dilute acid aqueous streams in the purification process to separate the acetic acid from dilute aqueous solutions to avoid discarding any acetic acid product with the water and provide a more controlled, purified aqueous stream which can be recycled or used effectively during purification of the acetic acid product.